Radio frequency transponders having the capability of storing information in their memories and communicating with radio frequency readers are known. These transponders typically have read/write or read-only memories and thus have the capacity to receive, store and transmit data upon command. Such transponders have, for example, been fixed to shipping tags so that containers being shipped can be tracked from their point of origin to their destination and points in-between. It will be appreciated that bar codes have frequently been used in the past for identifying documents, for example, shipping containers. Bar code identification labels, however, can become damaged and unreadable. They also require a direct line of sight with a bar code reader or scanner in order for the scanner to be effective. In contrast to bar codes, radio frequency transponders have the capability of standing up to various environments, for example, heat and handling, where twisting or bending of the substrate is common and do not require line of sight reading.
Typically, the transponders will contain an integrated circuit with a memory stage of at least one byte. The memory may be erasable or rewritable. Additionally, the transponder may comprise a chip-less type which contains a series of resonant circuits operating at different frequencies with each resonant circuit enabled or disabled corresponding to a unique binary number. The transponder may also be a passive (battery-less) type or an active type containing a battery. Transponders are also very typically of small size, e.g., can be 2.times.2 inches or less in planar form with a thickness of approximately 1/8-inch.